Silicon carbide (SiC) is promising as a semiconductor material more advantageous than silicon (Si). That is, if using silicon carbide for a semiconductor material, the withstand voltage is higher and the heat resistance is better than with silicon, so there are the advantages of a possibility of an increase in the voltage used, a reduction in loss, a reduction in size of the semiconductor chip, etc.
However, use for a semiconductor chip requires a high quality SiC single crystal greatly reduced in mozaicity, dislocation, and other faults. Further, from the viewpoints of the source material yield and production efficiency, it is important that large single crystals be able to be produced in a short time.
To obtain a single crystal of a compound which will not congruently melt (having no liquid phase state) like silicon carbide, the sublimation method and solution method are used.
The sublimation method is a method of causing a precipitating substance to subliminate at a high temperature part and precipitate from the vapor phase onto a seed crystal arranged at a low temperature part. The Acheson method, Lely method, and improved Lely method are representative methods of the same. However, since the substance is precipitated from a very thin phase, these are disadvantageous in the point of a low precipitation rate and, further, the problem of unavoidable formation of micropipe defects since the growth mechanism is a flank mechanism of spiral growth from the steps of the surroundings of spiral dislocations.
On the other hand, the solution method is a method of sufficiently dissolving a precipitating substance in a solvent at a high temperature part and causing a supersaturated state on a seed crystal arranged at a low temperature part to cause precipitation. The top-seeded solution growth (TSSG) method is representative of this. As one example of the solution method, Japanese Unexamined Patent Publication (Kokai) No. 2000-264790 discloses a method of dissolving a material including at least one type of transition metal element, silicon, and carbon by heating to form a melt and cooling this melt so cause precipitation and growth of a silicon carbide single crystal. According to the solution method, the solute concentration of the solvent can be adjusted, so the above problems of the vapor phase method can be solved, but the concentration gradient fluctuates according to the precipitated location due to the following reasons, so it is extremely difficult to obtain a uniform state of growth.
(1) The growth rate is still slow (0.1 mm/h or so). The growth rate can be accelerated by means such as making the concentration gradient near the precipitating parts sharp, but the precipitation state would become unstable and a high quality single crystal would not be able to be grown.
(2) The temperature gradient changes along with changes in the shape of the workpiece (heated object, including the source material (or crucible), solvent, seed crystal, support rod, etc.) or amount of source material charged. That is, in the temperature gradient, rather than control at the system side, the position and shape of the workpiec or heat source is a greater control factor, so obtaining a desired temperature gradient requires repeated calculation at the design stage and measurement of actual temperatures. Naturally, if the shape of the heated object is changed, the temperature gradient will change as well. The relative position with the heat source therefore has to be adjusted by trial and error.
(3) Achieving a uniform in-plane temperature distribution vertical to the crystal growth axis is difficult. The reasons are that any heating is from the outside surface of the workpiece and that the substance acting as the heat medium is a fluid such as a vapor phase or liquid phase and therefore convection affects the temperature distribution.
As a result, with a method of production of a silicon carbide single crystal by the conventional solution method, there were limits to improvement of the yield by increasing the size of the bulk single crystal.
On the other hand, silicon carbide single crystal is not only used in the bulk state. It also has a high value of use for semiconductor devices as the thin film formed on the surface of a seed crystal (so-called “epitaxial film”). In the past, in general, such a silicon carbide thin film has been formed by causing vapor phase growth by the chemical vapor deposition (CVD) method using silane and propane as the silicon source and carbon source. However, there were the defects of faults present in the seed crystal used as the substrate in the epitaxial growth from the vapor phase, in particular, in the case of silicon carbide, micropipes formed from the large Burgers vector hollow spiral dislocations, being passed along to the epitaxial growth film.
Therefore, with a silicon carbide single crystal thin film obtained by the conventional vapor phase growth, there were limits to the increase in quality through the reduction of faults.